Anaesthesia may be defined as a pharmacologically induced state of reversible unconsciousness, during which the patient neither perceives nor recalls noxious stimuli. Anaesthetic drugs depress all excitable tissues and the central neurons are amongst the most sensitive. At sufficient anaesthetic depth conscious awareness and recall are lost, and normal sensory, somatic and autonomic responses to surgical stimulation are absent.

Anaesthesia is a non-therapeutic intervention, performed within a dedicated environment. The complications of anaesthesia may be poorly tolerated, so all patients must be individually assessed to evaluate the benefits versus potential risks. The maintenance of maximum perioperative safety is of significant importance during the administration of anaesthesia.

Stages of clinical anaesthesia

In 1937, four stages of progressively deeper anaesthesia were described by Guedel in unpremedicated patients during inhalational induction with diethyl ether. Modern advances have resulted in considerable changes to techniques and available drugs, so that the early stages of anaesthesia often occur too rapidly to be easily distinguished. The stages may be seen in reverse on emergence from anaesthesia.

Analgesia

In stage 1, inhalational sedation occurs prior to the loss of the eyelash reflex and unconsciousness.

Excitement

During stage 2, the breathing gradually becomes more irregular and airway irritability increases. The pupils become more dilated and uncontrolled limb movements may occur. The eyelid reflex is lost.